Carbohydrate and free fatty acid utilization in skeletal muscle of trained and untrained men

Abstract:

In an effort to determine the relative amount of carbohydrates and free fatty acids (FFA) utilized by trained and untrained men at a submaximal workload and the relationship of selected mitochondrial and glycolytic enzyme activities, six trained and six untrained males were exercised for one hour at 50o of their maximal oxygen uptake (V02 max).Muscle samples were taken from the vastus lateralis in three cyclists and from the gastrocmemius in three runners from each group before and after exercise, and later assayed for succinic acid dehydrogenese, malic acid dehydrogenase, lactic acid dehydrogenase, phosphorylane, and carnitine paLmityltransf erase activities, and glycogen levels. Th e post exercise sample was assayed for glycogen, another was sectioned and stained for glycogen and fiber composition (PAS and-glycerolphosphate dehydrogenase) and from a third coupled mitochondria were isolated to determine 14'CO2 production from oxidation of varied levels of 14C-palmityl-CoA. Blood drawn before, immediately after, and thirty minutes after the exercise was assayed for FFA, glycerol, triglycerides, and glucose levels. Activities of oxidative enzymes (SDH, MDH, and CPT) were significantly higher (2-3 times greater) in the trained individuals than in the untrained group. Glycolytic enzyme activities were higher in the untrained group, probably due to higher fast twitch fiber populations. During exercise, FFA levels rose to the same degree in both groups; however, glycerol levels increased almost five times greater during the exercise in the trained subjects indicating the FFA turnover was much larger in the trained subjects. Blood glucose levels increased by an average of 11 mg% during the exercise in the trained subjects but fell by 8 mg% in the others, suggesting a greater selective uptake by the untrained subjects. Muscle glycogen depletion was 66% greater in the untrained group. These catabolic processes were independent of muscle fiber type, indicating that aerobic training increases those enzyme activities associated with FFA oxidation. Trained individuals are thus able to shift to fatty acids as the primary carbon source for the citric acid cycle, sparing glycogen during submaximal work.